One common application for thermal sensors is in thermal (infrared) imaging devices such as night vision equipment. One such class of thermal imaging devices includes a focal plane array of infrared detector elements or thermal sensors coupled to an integrated circuit substrate with a corresponding array of contact pads between the focal plane array and the integrated circuit substrate. The thermal sensors define the respective picture elements (or pixels) of the resulting thermal image.
One type of thermal sensor includes a ferroelectric or pyroelectric element formed from ferroelectric material that exhibits a state of electrical polarization dependent upon temperature changes in response to thermal radiation. An infrared absorber and common electrode are disposed on one side of the ferroelectric elements. A sensor signal electrode is disposed on the opposite side of each ferroelectric element. The infrared absorber and common electrode typically extends across the surface of the focal plane array and are attached to each of the ferroelectric elements. Each ferroelectric element typically has its own separate sensor signal electrode. Each infrared detector element or thermal sensor is defined, in part, by the infrared absorber and common electrode and the respective sensor signal electrode, which constitute capacitive plates, and the ferroelectric element, which constitutes a dielectric or insulator disposed between the capacitive plates.
To maximize thermal response and enhance thermal image accuracy, each ferroelectric element of the focal plane array is preferably isolated thermally from the integrated circuit substrate to insure that the sensor signal associated with each thermal sensor accurately represents incident infrared radiation. Thermal isolation structures are typically disposed between the focal plane array and the integrated circuit substrate to provide both mechanical bonding and a sensor signal flow path while minimizing thermal diffusion from the respective thermal sensor to the integrated circuit substrate.
Several approaches have been used to provide a thermal isolation structure between an array of thermal sensors and an underlying integrated circuit substrate. One approach is disclosed in U.S. Pat. No. 4,663,529 entitled Thermal Imaging Device and a Method of Manufacturing a Thermal Imaging Device to Jenner, et al., in which a square grid of channels form a corresponding grid of pillars that define thermal sensor elements. Each pillar or sensor includes a central bore that is coated with a conductive layer. The conductive bores are dimensioned to be less in diameter than corresponding electrode bumps disposed on an integrated circuit substrate, such that when the focal plane array of pillars is disposed over an integrated circuit substrate with a corresponding array of electrode bumps, the conductive bore of each thermal sensor rests on, and is electrically connected to, a corresponding electrode bump. A disadvantage of this architecture is that mating the array of conductive-bore pillars with the corresponding electrode bump array requires close tolerances and exact alignment. Another disadvantage of this architecture is that photoresist is used as the structural material for the pillars, which are therefore structurally fragile and susceptible to damage by solvents.
Another approach is disclosed in U.S. Pat. No. 4,143,269 entitled Ferroelectric Imaging System to McCormack, et al., assigned to Texas Instruments Incorporated, the assignee of this invention, which includes a thermal isolation structure for a thermal sensor array having conductive vias formed in a thermal isolation layer (polyimide) that covers an integrated circuit substrate. In this architecture, vias are formed in the thermal isolation layer, exposing contact pads on the integrated circuit substrate. The sensor signal electrode for a thermal sensor is brought into contact with a corresponding conductive via, providing an electrical connection to the associated contact pad. A disadvantage of this architecture is that so much polyimide is present that total thermal resistance is relatively low. In addition, this architecture requires a relatively large number of process steps, thereby increasing costs.
A further approach to providing a thermal isolation structure having thermal isolation mesas formed from polyimide is shown in U.S. Pat. No. 5,047,644 entitled Polyimide Thermal Isolation Mesa for a Thermal Imaging System to Meissner, et al., assigned to Texas Instruments Incorporation, the assignee of this invention.